A Technique for Optimal Core Buildups

INTRODUCTION
Recurrent caries at the margins of crowns pose unique restorative challenges. Until the old crowns are removed, the dentist cannot know if there remains sufficient tooth structure and adequate shapes for the new crowns. Core buildups for crowns may be necessary to properly restore these teeth (Figures 1 and 2).

Ideally, a core buildup done for a crown should have a total occlusal convergence (TOC) between 6° and 12° and an axial wall height of at least 3 to 4 mm. A TOC less than 6° to 12° may introduce unnecessary challenges to seating. Conversely, a TOC of greater than 20° may be detrimental to retention of the final restoration. The aspect of the preparation that most requires the optimal TOC is in the cervical one third. Further, the restorative needs of the final crown dictate the amount of occlusal reduction for the preparation, with adequate dimension for structural integrity and for clearance or guidance in excursive movements.

When removing a failed restoration, the dentist may encounter remaining tooth dimensions that lack some or all of these criteria, or worse, decimated teeth. There may be multiple teeth that require buildups to create the right shape and form. Dentists generally address these important considerations by over-bulking the build-up material and then trimming it back in an ad-lib technique that can create a less than ideal result.

The dental laboratory team can help to overcome these challenges by preparing the full-contour wax-up and creating a silicone matrix for chairside fabrication of optimally shaped core buildups for crowns. Using wax-ups to create a matrix for fabricating provisional restorations is widely used. Dentists enjoy using these matrices because they incorporate the artistry and skill of the laboratory technician in helping to develop functional and beautiful provisional restorations. This completed diagnostic wax-up is the starting point for this technique (Figure 3).

Technique for Creating a Lab-Fabricated Build-Up Matrix
In the dental laboratory, the technician creates a full contoured wax-up on articulated models that best mimics the needs of the final restorations. A silicone matrix of the wax design is made for the dentist to use chairside for making the provisionals. With the final, articulated wax design still on the lab bench, the technician implements an additional step: the full-contoured wax-ups are then cut back (“prepared”) on the articulator to mimic ideal intraoral preparations yet to be done by the clinician for the final crown preparations, taking into consideration the functional and esthetic needs of the proposed final crown, such as taper, wall heights, occlusal clearance, and guidance (Figure 4).

At the chair, the dentist removes the failed crowns, excavates any caries, and prepares the teeth with etchant and bonding. The build-up matrix is filled with build-up material (composite resin core material) and placed onto the preparations and allowed to set. After complete chemical set, the matrix is removed and the buildups are firmly bonded to the teeth. Any excess flash is removed and the preformed preparations (that are duplicates of the wax cutback “preparations”) are simply refined (Figures 5 and 6).

As seen in Figures 1 to 6, the use of crown build-up matrices for posterior work has the obvious benefit of efficiency of fabrication. But an even more impressive use of this technique is in establishing optimal clearance for anterior guidance in anterior restorative cases.

The following case report will describe further details of the technique.

Figure 1. Preoperative view.	Figure 2. After caries control.
Figure 3. Full-contour wax-up.	Figure 4. Prepared wax-up.
Figure 5. Chemically set buildups.	Figure 6. Lightly prepared buildups.

CASE REPORT
Diagnosis and Treatment Planning
This 78-year-old patient sought restorative treatment for recurrent caries of teeth Nos. 6 to 11 (Figure 7). She had 4 mm to 5 mm pocketing with no signs of inflammation. Records were performed and articulated models were created and sent to the laboratory. The technician created a full-contour wax-up that addressed the functional and esthetic needs of the final restorations (Figure 8).

It was clear from the preliminary study of the articulation that only a shallow guidance would be needed to disclude the posterior teeth in excursions. A custom incisal guide table was made to register the anterior guidance that was so carefully designed and developed in the wax-up (Figure 9). The custom incisal guide table plays a major role in this process, since it guides the technician to accurately assess the amount of clearance necessary for each preparation in centric, canine-rise, crossover, and protrusive excursions. If clearance for guidance was left to be performed only clinically (and therefore, without the benefit of the custom incisal guide table), the reference of the planned anterior guidance and amount of tooth reduction necessary for each tooth would be lost.

Dental Laboratory Protocol
Before preparing the wax, the technician duplicated the delicate wax-up model with a model duplicating material (SternTek Duplicating Material [Sterngold]). SternTek+ is an extremely precise, pourable, duplicating material based on addition curing silicones. The SternTek mold was poured with lab stone and was used as a model to create the provisional matrix as well as other matrices to guide in the reduction of the wax-ups (Figure 10).

Next, the full-contour wax designs were prepared (cut back), and reshaped to mimic the proposed final core buildups (completed crown preparations). The wax-ups were shaped one at a time, using the adjacent teeth, laboratory matrices, and depth cuts as a reference for wax reduction. The dental technician created these crown preparation shapes with a contemplative and deliberate process, free from the pressure and stress of clinical crown reduction (Figures 11 and 12). A new silicone matrix was made of the prepared wax-ups for chairside fabrication of the buildups (Figure 13).

Figure 7. Anterior pre-op view.	Figure 8. Full-contour wax-up.
Figure 9. Custom incisal guide table.	Figure 10. Matrix for provisionals.
Figure 11. Prepared wax-ups.	Figure 12. Clearance for guidance.
Figure 13. Matrix for core buildups.	Figure 14. After old crown removal.

Then, the technician delivered both matrices to the dentist: one for fabricating the ideal core buildups, and the other to fabricate the provisionals.

Clinical Protocol
The failed crowns were removed and the decay excavated. The previous crown, cement, caries, and excavation process leaves the tooth structure highly contaminated (Figure 14). To decontaminate the tooth, the author uses the basic techniques described by Dr. William Strupp.¹ The remaining tooth structure is lightly prepared with a diamond and retraction cords placed to isolate the field (No. 7 SilTrax Epi [Pascal International]) (Figure 15). The tooth is then sequentially scrubbed and rinsed with the following agents: 4% chlorhexidine gluconate (HIBICLENS [Molnlycke Health Care]) to disinfect; Tubulicid Red (Global Dental Products) to remove the smear layer; sodium hypochlorite (Clorox) for final disinfection.¹

The bonding process is started with: Etchant with 32% H3PO4 (32% Uni-Etch w/BAC [BISCO Dental Products]), Primer (ALL-BOND 3 Primer [BISCO Dental Products]), and Resin (ALL-BOND 3 Resin [BISCO Dental Products]). Core Paste XP (DenMat) is well suited for the build-up material. It is a dual-cured, fluoride releasing, low viscosity material that works well with the matrix technique described.¹

When the teeth are ready for the buildup, the inside of the build-up matrix is filled with the Core Paste XP material (Figure 16), and a small amount is syringed directly onto the preparations. When filling the build-up matrix with the Core Paste XP, fill only the individual preparation areas of the matrix (compared to filling a provisional matrix contiguously from the most mesial tooth to the most distal). The matrix is then completely seated and allowed to set, undisturbed, for the duration of time established by the manufacturer for a complete chemical cure.

Figure 15. Teeth prepared and ready for buildups.	Figure 16. Injecting Core Paste XP (DenMat) into the silicone matrix.
Figure 17. Following matrix removal.	Figure 18. After high-speed finishing.

Figure 19. Final crowns at try-in.

After the material has completely set, the matrix is removed, leaving the cured crown buildups with their preplanned forms firmly bonded to the teeth (Figure 17). Any remnant or excess flash is removed and the buildups are lightly refined with high-speed diamonds (Figure 18). The restorative process then continues through master impressions, fabrication of the temporaries (with the lab-fabricated provisional matrix), and delivery of final crowns (Figure 19).

DISCUSSION
For the dentist who utilizes preoperative articulated models and wax-ups, this technique contributes to efficient fabrication of core buildups for crowns. This technique requires no additional dentist time to implement, as the articulation and full-contour wax-ups are already in the laboratory. In this way, the knowledge and skill of the dental technician is used to guide the design of the final build-up shapes. The technician is able to capture and send digital views of the full-contour wax designs and e-mail them to the dentist for approval of the esthetics and guidance, followed by a second communication of photos of the core build-up design both in centric as well as excursion movements.

As with all prosthodontic treatment, the outcome depends on the accuracy of the initial articulation. Creating quality, articulated models will result in greater accuracy of all the steps that follow: the trial equilibration, diagnostic wax-up, provisional matrix, reduction of the wax-up for crown buildups, the crown build-up matrix, accuracy in creating crown buildups at chairside, and in fabricating esthetic and functional provisional restorations.

While accurate impressions can be made with alginate, they have limitations that can be overcome by using a vinyl polysiloxane (alginate substitute) impression material for use in diagnostic models. This impression material can be poured by the technician several times for legal records, trial equilibration, and for diagnostic articulation. The impression technique should capture all details of the full arch of teeth, the cervical margins of the teeth, as well as the adjacent soft tissue to register the landmarks that will be used in seating the silicone matrices intraorally.

IN SUMMARY
The amount of viable tooth structure remaining underneath failed full-coverage restorations (crowns) is always an unknown. Dentists are often faced with nearly destroyed teeth that can border on the need for endodontic therapy to facilitate restoration.

The benefits of the technique described in this article are:

• Preoperatively designed core build-up shapes with a matrix
• Preoperative planning for optimal reduction for anterior guidance cases
• Reduced chairside time and reduced stress in restoring very damaged teeth
• No additional dentist time in implementation of the process.

Having a well planned, laboratory-designed build-up matrix available can turn a dismal clinical discovery into a calm and efficient restorative visit.

Acknowledgement
The authors wish to acknowledge the seminars and techniques of Dr. William Strupp, who carefully outlines the detailed process of creating crown buildups.

References

1. Strupp WC. Strupp Technique Manual 2012. Clearwater, FL: William C. Strupp, Jr; 2012.

Dr. Montrose practices dentistry in Skokie, Ill. He can be reached at (847) 902-8906 or at jmontrosedds@gmail.com.

Mr. Bartosiak is the owner and ceramist of North American Dental Laboratory in Skokie, Ill. He can be reached at (847) 982-9788 or at nadlinc@yahoo.com.

Disclosure: The authors report no disclosures.